On the Effects of Interstitial Elements on Microstructure and Properties of Ternary and Quaternary TiAl based alloys
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On the Effects of Interstitial Elements on Microstructure and Properties of Ternary and Quaternary TiAl based alloys Jean-Pierre Chevalier1, 2, Mélanie Lamirand1 and Jean-Louis Bonnentien1 1 Centre d'Etudes de Chimie Métallurgique, Centre National de la Recherche Scientifique, 15, rue Georges Urbain, F 94407 Vitry cedex 2 also at Chaire des Matériaux Industriels Métalliques et Céramiques, Conservatoire National des Arts et Métiers, 2, rue Conté F 75003 Paris ABSTRACT Ti-Al-Cr ternary and Ti-Al-Cr-Nb quaternary alloys have been studied as a function of initial purity and added interstitial content. Using strict clean processing together with either ultra high purity or commercial purity alloys, the effects of interstitial elements (essentially O, but also C and N) on microstructure and hardness, yield stress and fracture strain have been studied for both fully lamellar microstructures and duplex microstructures. The results are clear and similar trends are observed : as long as they do not precipitate, these stabilise the lamellar microstructure and affect the kinetics of the α-γ phase tranformation, leading to a higher than equilibrium value for the α2 phase for continuous cooling. Both the lamellar spacing and the α2 phase fraction correlate with increased hardness and yield stress, and also with decreasing fracture strain. The effects are significant. INTRODUCTION It is well known that titanium and its alloys can have notable levels of interstitial elements in solid solution [1]. Furthermore, the alloying reaction for the formation of Ti-Al based alloys is strongly exothermic, leading to a further increase in oxygen content of the resulting alloy via contamination of the molten alloy. For example the oxygen level of alloys studied are frequently around 1000 wt ppm. The effects of interstitial elements on phase stability, microstructrure and properties have been studied previously (e.g. [2, 3]), but notably less than the effects of substitutional alloying elements, probably due to the experimental difficulties of very clean processing, and use of high purity starting materials. Our previous work on the effect of O, N and C content on binary Ti-Al alloys showed clear trends [4]. After continuous cooling leading to fully lamellar microstructures, the volume fraction of α2 phase (Fvα2) increased as a function of interstitial content, up to interstitial concentrations where precipitation of oxides, nitrides or carbides could be detected. This led to a larger number of α2 lamellas, with a concurrent decrease in the interlamellar spacing (i.e. the spacing corresponding to the different variants of γ lamellas). In terms of mechanical properties, hardness increased with Fvα2 and the decrease in α2 lamellar spacing. The aim of the study summarised here was to ascertain :
S6.11.2
- whether the addition of ternary (Cr) or quaternary (Cr and Nb) substitutional elements alters the trends previously observed ; - what is the effect O, C and N on fracture strain (taken as an indication of ductility) ; - whether the eff
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